Term introduced by Hare to describe the way that ethical properties relate to other, psychological and natural properties of things. Properties of one kind, F, supervene upon those of another kind, G, when things are F in virtue of being G. Thus a person cannot just be good, but must be good in virtue of possessing other properties, such as courage or kindness. The supervening property relates to the underlying qualities in at least this way: if one thing possesses the underlying properties and is F, then any other thing with just the same underlying properties must share the resultant property F. Sometimes a distinction is made between weak supervenience, where this is held only of other actual things, and strong supervenience, where it applies to all possible things.

The notion is exploited in many areas: for example, biological properties plausibly supervene upon chemical ones; mental properties upon physical ones; dispositional properties and powers upon categorical ones, and so on. One promise the notion holds out is that by its means we can understand the relation of such different layers of description without attempting a reduction of the one area to the other. The value of this promise depends on how well we understand the supervenience relation itself. If it is a dangling, inexplicable, metaphysical fact that the Fs relate in this way to the Gs, then supervenience inherits rather than solves the problems of understanding the various areas. It has been argued that in ethics supervenience is well explained by expressivist theories but not at all by realist or intuitionist theories, on which moral properties might be wandering stars in the metaphysical firmament. See also variable realization.